The menstrual cycle, fertilization and the embryo

In placental mammals the ovary becomes active regularly, in cycles linked with mating and reproduction. This is the breeding cycle. It differs between species: in dogs it comes about twice a year, and in rats every few days. In women it is the menstrual cycle. A typical cycle is 28 days, but cycles differ between people, and a doctor is the person to ask about any concern. The cycle has three phases. In the egg-maturing stage the pituitary releases FSH. FSH makes a follicle grow into a mature Graafian follicle, and the egg matures in it in about 10 days. The follicle makes oestrogen, which builds up the endometrium. Only one follicle matures and releases an egg in each cycle. In the ovulation stage, at about day 14, the pituitary releases LH, and the follicle releases the egg. The follicle becomes the corpus luteum, which makes oestrogen and progesterone. These two hormones thicken the endometrium and increase its blood supply for about 14 days. In the menstruation stage, if the egg is not fertilized, the corpus luteum slowly degenerates. The hormones fall, the endometrium breaks down and bleeds for 3 to 5 days, and a new cycle starts. If the egg is fertilized, pregnancy begins. The corpus luteum stays and keeps making oestrogen and progesterone, which stop ovulation, so the cycle pauses until after birth. The corpus luteum is largest at the end of the third month and shrinks in the fourth month, when the placenta takes over making oestrogen and progesterone. Progesterone also prepares the breasts. If the corpus luteum is lost early in pregnancy, before the placenta has taken over, a miscarriage can follow.